We report on the sensitivity of enhanced ozone (O₃) production, observed during lake breeze circulation along the coastline of Lake Michigan, to the concentrations of nitrogen oxides (NO_x = NO + NO₂) and volatile organic compounds (VOCs). We assess the sensitivity of O₃ production to NO_x and VOC on a high O₃ day during the Lake Michigan Ozone Study 2017 using an observationally constrained chemical box model that implements the Master Chemical Mechanism (MCM v3.3.1) and recent emission inventories for NO_x and VOCs. The Master Chemical Mechanism model is coupled to a backward air mass trajectory analysis from a ground supersite in Zion, IL, where an extensive series of measurements of O₃ precursors and their oxidation products, including hydrogen peroxide (H₂O₂), nitric acid (HNO₃), and particulate nitrates (NO₃⁻) serve as model constraints. We evaluate the chemical evolution of the Chicago‐Gary urban plume as it advects over Lake Michigan and demonstrate how modeled indicators of VOC‐ versus NO_x‐sensitive regimes can be constrained by measurements at the trajectory endpoint. Using the modeled ratio of the instantaneous H₂O₂ and HNO₃ production rates (P_H2O2/P_HNO3), we suggest that O₃ production over the urban source region is strongly VOC sensitive and progresses towards a more NO_x‐sensitive regime as the plume advects north along the Lake Michigan coastline on this day. We also demonstrate that ground‐based measurements of the mean concentration ratio of H₂O₂ to HNO₃ describe the sensitivity of O₃ production to VOC and NO_x as the integral of chemical production along the plume path.

The exchange of trace gases between the biosphere and the atmosphere is an important process that controls both chemical and physical properties of the atmosphere with implications for air quality and climate change. The terrestrial biosphere is a major source of reactive biogenic volatile organic compounds (BVOCs) that govern atmospheric concentrations of the hydroxy radical (OH) and ozone (O₃) and control the formation and growth of secondary organic aerosol (SOA). Common simulations of BVOC surface–atmosphere exchange in chemical transport models use parameterizations derived from the growing season and do not consider potential changes in emissions during seasonal transitions. Here, we use observations of BVOCs over a mixed temperate forest in northern Wisconsin during broadleaf senescence to better understand the effects of the seasonal changes in canopy conditions (e.g., temperature, sunlight, leaf area, and leaf stage) on net BVOC exchange. The BVOCs investigated here include the terpenoids isoprene (C₅H₈), monoterpenes (MTs; C₁₀H₁₆), a monoterpene oxide (C₁₀H₁₆O), and sesquiterpenes (SQTs; C₁₅H₂₄), as well as a subset of other monoterpene oxides and dimethyl sulfide (DMS). During this period, MTs were primarily composed of α-pinene, β-pinene, and camphene, with α-pinene and camphene dominant during the first half of September and β-pinene thereafter. We observed enhanced MT and monoterpene oxide emissions following the onset of leaf senescence and suggest that senescence has the potential to be a significant control on late-season MT emissions in this ecosystem. We show that common parameterizations of BVOC emissions cannot reproduce the fluxes of MT, C₁₀H₁₆O, and SQT during the onset and continuation of senescence but can correctly simulate isoprene flux. We also describe the impact of the MT emission enhancement on the potential to form highly oxygenated organic molecules (HOMs). The calculated production rates of HOMs and H₂SO₄, constrained by terpene and DMS concentrations, suggest that biogenic aerosol formation and growth in this region should be dominated by secondary organics rather than sulfate. Further, we show that models using parameterized MT emissions likely underestimate HOM production, and thus aerosol growth and formation, during early autumn in this region. Further measurements of forest–atmosphere BVOC exchange during seasonal transitions as well as measurements of DMS in temperate regions are needed to effectively predict the effects of canopy changes on reactive carbon cycling and aerosol production.

Oceans emit large quantities of dimethyl sulfide (DMS) to the marine atmosphere. The oxidation of DMS leads to the formation and growth of cloud condensation nuclei (CCN) with consequent effects on Earth’s radiation balance and climate. The quantitative assessment of the impact of DMS emissions on CCN concentrations necessitates a detailed description of the oxidation of DMS in the presence of existing aerosol particles and clouds. In the unpolluted marine atmosphere, DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a stable intermediate in the chemical trajectory toward sulfur dioxide (SO₂) and ultimately sulfate aerosol. Using direct airborne flux measurements, we demonstrate that the irreversible loss of HPMTF to clouds in the marine boundary layer determines the HPMTF lifetime (τ_HPMTF < 2 h) and terminates DMS oxidation to SO₂. When accounting for HPMTF cloud loss in a global chemical transport model, we show that SO₂ production from DMS is reduced by 35% globally and near-surface (0 to 3 km) SO₂ concentrations over the ocean are lowered by 24%. This large, previously unconsidered loss process for volatile sulfur accelerates the timescale for the conversion of DMS to sulfate while limiting new particle formation in the marine atmosphere and changing the dynamics of aerosol growth. This loss process potentially reduces the spatial scale over which DMS emissions contribute to aerosol production and growth and weakens the link between DMS emission and marine CCN production with subsequent implications for cloud formation, radiative forcing, and climate. Significance Ocean emissions of dimethyl sulfide (DMS) are a major precursor for the production and growth of aerosol particles, which can act as seeds for the formation of cloud droplets in the marine atmosphere with the subsequent impacts on Earth’s climate. Global aircraft observations indicate that DMS is efficiently oxidized to hydroperoxymethyl thioformate (HPMTF), a previously unrecognized molecule, which necessitates revisiting DMS oxidation chemistry in the marine atmosphere. We show through ambient observations and global modeling that a dominant loss pathway for HPMTF is uptake into cloud droplets. This loss process short circuits gas-phase oxidation and significantly alters the dynamics of aerosol production and growth in the marine atmosphere.

Aerosol particles influence global climate by determining cloud droplet number concentrations, brightness, and lifetime. Primary aerosol particles, such as those produced from breaking waves in the ocean, display large particle–particle variability in chemical composition, morphology, and physical phase state, all of which affect the ability of individual particles to accommodate water and grow into cloud droplets. Despite such diversity in molecular composition, there is a paucity of methods available to assess how particle–particle variability in chemistry translates to corresponding differences in aerosol hygroscopicity. Here, an approach has been developed that allows for characterization of the distribution of aerosol hygroscopicity within a chemically complex population of atmospheric particles. This methodology, when applied to the interpretation of nascent sea spray aerosol, provides a quantitative framework for connecting results obtained using molecular mimics generated in the laboratory with chemically complex ambient aerosol. We show that nascent sea spray aerosol, generated in situ in the Atlantic Ocean, displays a broad distribution of particle hygroscopicities, indicative of a correspondingly broad distribution of particle chemical compositions. Molecular mimics of sea spray aerosol organic material were used in the laboratory to assess the volume fractions and molecular functionality required to suppress sea spray aerosol hygroscopicity to the extent indicated by field observations. We show that proper accounting for the distribution and diversity in particle hygroscopicity and composition are important to the assessment of particle impacts on clouds and global climate.

We describe the characterization and field deployment of chemical ionization mass spectrometry (CIMS) using a recently developed focusing ion-molecule reactor (FIMR) and ammonium–water cluster (${\mathrm{NH}}_{\mathrm{4}}^{+}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ ) as the reagent ion (denoted as ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS). We show that ${\mathrm{NH}}_{\mathrm{4}}^{+}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ is a highly versatile reagent ion for measurements of a wide range of oxygenated organic compounds. The major product ion is the cluster with ${\mathrm{NH}}_{\mathrm{4}}^{+}$ produced via ligand-switching reactions. Other product ions (e.g., protonated ion, cluster ion with ${\mathrm{NH}}_{\mathrm{4}}^{+}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ , with H₃O⁺, and with ${\mathrm{H}}_{\mathrm{3}}{\mathrm{O}}^{+}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ ) are also produced, but with minor fractions for most of the oxygenated compounds studied here. The instrument sensitivities (ion counts per second per part per billion by volume, cps ppbv⁻¹) and product distributions are strongly dependent on the instrument operating conditions, including the ratio of ammonia (NH₃) and H₂O flows and the drift voltages, which should be carefully selected to ensure ${\mathrm{NH}}_{\mathrm{4}}^{+}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ as the predominant reagent ion and to optimize sensitivities. For monofunctional analytes, the ${\mathrm{NH}}_{\mathrm{4}}^{+}\cdot {\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ chemistry exhibits high sensitivity (i.e., >1000 cps ppbv⁻¹) to ketones, moderate sensitivity (i.e., between 100 and 1000 cps ppbv⁻¹) to aldehydes, alcohols, organic acids, and monoterpenes, low sensitivity (i.e., between 10 and 100 cps ppbv⁻¹) to isoprene and C1 and C2 organics, and negligible sensitivity (i.e., <10 cps ppbv⁻¹) to reduced aromatics. The instrumental sensitivities of analytes depend on the binding energy of the analyte–${\mathrm{NH}}_{\mathrm{4}}^{+}$ cluster, which can be estimated using voltage scanning. This offers the possibility to constrain the sensitivity of analytes for which no calibration standards exist. This instrument was deployed in the RECAP campaign (Re-Evaluating the Chemistry of Air Pollutants in California) in Pasadena, California, during summer 2021. Measurement comparisons against co-located mass spectrometers show that the ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS is capable of detecting compounds from a wide range of chemical classes. The ${\mathrm{NH}}_{\mathrm{4}}^{+}$ CIMS is valuable for quantification of oxygenated volatile organic compounds (VOCs) and is complementary to existing chemical ionization schemes.